Conventionally, diversity receiving devices are available to receive broadcast waves stably when broadcast receiving devices are on the move. As a method for reducing desensitization due to self noise in a receiver, a noise cancelling technique is disclosed in Patent Literature 1. According to this technique, the noise is first processed to have an opposite phase and an appropriate amplitude, and then injected into the reception circuit so as to cancel the noise.
FIGS. 10 to 12 show a wireless transceiver disclosed in Patent Literature 1. FIG. 10 shows wireless communication terminal 700 having two antennas as an example of the wireless transceiver. In wireless communication terminal 700, signals are received by receiving antennas 601 and 602, demodulated by receivers 701 and 702, respectively, and then transmitted to interference canceller 703. Interference canceller 703 removes radiation noise components from the signals, and outputs signals containing no radiation noise to information processing system 704.
FIG. 11 shows a first example of interference canceller 703 of FIG. 10. This example is effective when the interference signals received by receiving antennas 601 and 602 are highly correlated with each other. Receiver 701 demodulates a signal received by receiving antenna 60 1 and outputs signal 705, which is the sum of a desired signal and an interference signal. Receiver 702, on the other hand, demodulates a signal received by receiving antenna 602 and outputs signal 706, which is the sum of a desired signal and an interference signal. Signal 706 is outputted to amplitude-phase regulating unit 802, which controls the interference signal contained in signal 706 to be equal in amplitude and phase to the interference signal contained in signal 705. Amplitude-phase regulating unit 802 then outputs regulated signal 708 to addition unit 803, which adds signal 708 with signal 704, and then outputs signal 710.
In the default setting, the interference signal contained in signal 710 outputted from addition unit 803 is monitored without transmitting data. Since signal 710 does not contain data in this condition, amplitude-phase regulating unit 802 sets the electric power of the interference signal obtained by adding signals 705 and 708 to be substantially zero. This default setting allows interference canceller 703 to operate stably and hence to attenuate radiation noise with increasing correlation between the interference signals received by receiving antennas 601 and 602.
FIG. 12 shows another example of interference canceller 703 of FIG. 10. In this example, interference canceller 703 receives radiation noise directly from an electronic device, a PC 914 (Personal Computer) via cable 912. Interference canceller 703 then inputs the radiation noise information to amplitude-phase regulating units 904 and 905 to generate pseudo-interference signals 728 and 730 of the receiving systems. The outputs of amplitude-phase regulating units 904 and 905 are respectively inputted to second and first addition units 902 and 901 as pseudo-interference signals 728 and 730, which become the output signals of radiation noise predictor 903. First and second addition units 901 and 902 subtract pseudo-interference signals 730 and 728, respectively, from the received signals. As a result, first and second addition units 901 and 902 output data 732 and 734, respectively, from which the radiation noise has been removed. Data 732 and 734 are then received by diversity reception device 910.
These conventional devices using noise-cancelling and diversity techniques, however, are large in size and usually mounted on vehicles such as cars, thus being not suitable to be reduced to a portable size. When reduced to a portable size, these devices cause a decrease in the receiving sensitivity due to the coupling between diversity antennas and also cause gain degradation due to the diversity correlation. This makes it impossible for these devices to provide desired high-sensitivity reception.
When a device includes a plurality of noise sources, such as a signal processing circuit, a clock part, a liquid crystal display driving part, bus lines in various types of memories, and a DC-DC converter, it is difficult to identify the noise component causing the gain degradation in each channel. Therefore, cancelling the noise component causing the gain degradation requires picking up all the related noise components.
As described above, in the technique disclosed in Patent Literature 1, the radiation noise is directly received from the electronic device, the PC 914 as a noise source via cable 912. Then, amplitude-phase regulating units 904 and 905 generate pseudo-interference signals of the receiving systems, using the received radiation noise information. The outputs of amplitude-phase regulating units 904 and 905 are inputted to second and first addition units 902 and 901 as pseudo-interference signals 728 and 730, which become the output signals of radiation noise predictor 903. Thus, first and second addition units 901 and 902 subtract pseudo-interference signals 730 and 728, respectively, from the received signals.
Therefore, the technique disclosed in Patent Literature 1 is effective to the noise component of the electronic device, the PC 914, but does not act on the other noise sources inside. Thus, this technique does not reduce desensitization caused by their noises.
When the technique is applied to a small-sized device, the device causes a decrease in the receiving sensitivity due to the coupling between diversity antennas and also causes gain degradation due to the diversity correlation. Thus, the problem of not having the ability to provide high-sensitivity reception remains unsolved.
Patent Literature 1: Japanese Patent Unexamined Publication No. 2004-236171